The present invention relates to a phase-locked loop (PLL), and more particularly, to a PLL for generating an output signal according to an input signal, wherein the frequency of the output signal is at least twice of the frequency of the input signal.
Phase-locked loop (PLL) has many applications, such as clock/data recovery, frequency or phase modulation/demodulation, and generating clocks with stable frequency so that PLLs are widely applied in various electronic devices, consumer products, and communication devices. In general, a conventional PLL comprises a phase frequency detector (PFD) for detecting phase error and frequency error between an input signal and a feedback signal, and a loop filter for adjusting the operation of a VCO according to the detection result of the PFD until the frequency and phase of the feedback signal match that of the input signal.
In a conventional recordable optical disk drive, for example, a PLL is used to generate a write clock according to a wobble signal. Generally, the conventional recordable optical disk drive utilizes a pick-up head to emit a laser beam to the wobbled groove on a recordable optical disk. Then, the pick-up receives the signal reflected from the recordable optical disk, and a push-pull signal is extracted from the reflected signal. As is well known in the art, a level slicer and a band pass filter can be employed to extract a wobble signal, which corresponds to the physical address of the optical disk, from the push-pull signal. In DVD−R/RW format, a wobble period corresponds to 186 periods of the write clock. In DVD+R/RW format, a wobble period corresponds to 32 periods of the write clock. When writing data into the recordable optical disk, the conventional recordable optical disk drive typically uses one write clock period as a minimum length unit. Therefore, in order to write data into the correct track, the PLL is employed in the conventional recordable optical disk drive to synchronize the write clock and the wobble signal.
Please refer to FIG. 1, which shows a schematic diagram of a wobbled groove and a corresponding wobble signal according to the related art. In a DVD+R/RW disk, as is well known in the art, the address information is recorded in the wobbled groove with the phase changes of wobble. Accordingly, the wobble signal generated based on the wobbled groove also has some periods with phase-change. For example, when the phase of the wobbled groove is suddenly changed, it possibly generates some signal impulses 102, 104, 106, etc. as shown in FIG. 1.
However, since the PFD employed in the conventional PLL is very sensitive to the edges of the foregoing impulses, it easily operates incorrectly. Furthermore, noise of the wobble signal is also probably causes the PFD to operate incorrectly and thereby affects the performance of the PLL.